1. Field of the Invention
The present invention relates to a catalyst for purifying an exhaust gas and a method for purifying an exhaust gas. More specifically, it relates to a catalyst for purifying an exhaust gas and a method for purifying an exhaust gas which can effectively remove nitrogen oxides (NOx) from an exhaust gas under an oxygen rich atmosphere, that is, an exhaust gas having an oxygen content larger than that necessary for completely oxidizing reducible substances contained in an exhaust gas, such as carbon monoxide, hydrogen and hydrocarbons i.e., larger than a stoichimetrical air-fuel ratio.
2. Description of the Related Art
Many three-way catalysts for purifying an exhaust gas are known as a catalyst for purifying an exhaust gas discharged from automobiles. These three-way catalysts are capable of oxidizing carbon monoxide (CO) and hydrocarbons (HC) contained in an exhaust gas and simultaneously reducing nitrogen oxides (NOx) co-existing in the exhaust gas. Typical examples of this type of catalyst include a catalyst prepared by coating a .gamma.-alumina slurry on a heat-resistant carrier, such as cordierite, calcining the coated carrier and supporting noble metals, such as Pd, Pt and Rh, on the carrier.
The performance of these catalysts for purifying an exhaust gas is greatly influenced by the set air-fuel ratio of engines. Particularly, the oxygen content of the exhaust gas after burning becomes large in a lean mixture, that is, on the lean side having a large air-fuel ratio, so that the oxidation process becomes active while the reduction process becomes inactive. On the other hand, on the rich side having a small air-fuel ratio, the oxygen content of the exhaust gas after burning becomes so small that the oxidation process becomes inactive, while the reduction process becomes active.
On the other hand, in recent years, energy saving is required from the viewpoint of resource problems and an increase in the temperature of the earth, which has led to a strong demand for lowering the fuel consumption of automobiles. In the gasoline engine, lean burn has attracted attention from the viewpoint of efficient burning. In the lean burn, the burning during ordinary running of automobiles is effected on the lean side as much as possible, wherein an oxygen-rich air-fuel mixture is burned. For this reason, the development of a catalyst, which can sufficiently remove NOx even on the lean side, has been desired in the art.
Accordingly, in order to solve the above-mentioned problems, various catalysts for purifying an exhaust gas have been proposed as a catalyst for purifying an exhaust gas discharged from automobiles under the above-mentioned oxygen rich atmosphere, wherein the oxidation of carbon monoxide, hydrocarbons, etc., and the reduction of nitrogen oxides are simultaneously effected.
For example, a proposal has been made regarding a Pt/Al.sub.2 O.sub.3 catalyst comprising an alumina carrier and platinum supported on the carrier, which catalyst is simultaneously capable of oxidizing carbon monoxide and hydrocarbons and of reducing nitrogen oxides.
We have previously proposed "Method for Purifying Exhaust Gas" wherein an oxygen rich exhaust gas is brought into contact with a catalyst comprising a porous material and, supported thereon, platinum and lanthanum (see Japanese Patent Application No. 3-344781).
In the former Pt/Al.sub.2 O.sub.3 catalyst, however, the conversion of NOx under an oxygen rich atmosphere is at most 20 to 30% (air-fuel ratio: A/F=about 22 to 23) even when the amount of the supported platinum is increased, and therefore, it has a problem that the conversion of NOx is unsatisfactory for practical use of the catalyst.
On the other hand, the latter catalyst disclosed in Japanese Patent Application No. 3-344781 can enhance the activity for reacting hydrocarbons with NOx by virtue of the addition of lanthanum to the catalyst to improve the transient properties in the transition: of the lean region.fwdarw.the stoichiometrical region.fwdarw.the lean region, and further can improve the conversion of NOx at the time of acceleration and deceleration through the utilization of the capability of lanthanum oxide to adsorb NOx. However, the effect obtained by the use of lanthanum is still unsatisfactory. Further, there is a problem in this catalyst that lanthanum may deteriorate the durability of the catalyst.